Machines for tunneling upwardly in a mine are well-known in the art, and are sometimes referred to as raise boring machines, or box hole machines, but in the present description this type of machine is referred to as a vertical thrust boring machine.
Among the many prior art U.S. patents are the Busby U.S. Pat. No. 3,830,318 and the Crane et al. U.S. Pat. No. 3,840,272.
It has been customary in machines of this type to utilize a holding means, sometimes referred to as a hairpin assembly, which grasps the drill string above the elevation of the lowermost drill stem section so as to establish the vertical position of the drill string while the lowermost section is being attached or detached. When the tunnel is being extended upwardly the drill stem sections are successively attached to the lower end of the drill string so as to extend the length of the string. When the cutter head is being retrieved the drill stem sections are successively detached, one at a time, from the lower end of the string.
Many and perhaps most of the vertical thrust boring machines have made up the drill string by using drill stem sections having threaded ends, there being a male thread on one end and a female thread on the other end. The attachment or detachment of a drill stem section, therefore, requires a rotating drive for the purpose of either making or breaking a threaded connection.
Experience has shown that when operating this type of machine in a mine, the "down time" is a significant problem. That is, a substantial portion of the working time of both the machine and its operator is used up in attaching or detaching the pipe sections. This operation is a rather delicate and difficult one, because while the threads on the pipe have a definite pitch which establishes a predetermined ratio between rotary movement and vertical movement, the machines have customarily been provided with a vertical drive for the rotary platform which is separate and apart from the rotary drive for the rotary platform. Furthermore, there does not appear to be any convenient means of synchronizing the vertical drive with the rotary drive, that would be reliable under the operating conditions to which the machine is subjected.
Therefore, the "down time" involved in attaching or detaching pipe sections to the drill stem has been accepted as a necessary evil or limitation of this type of machine.